DE3872208T2 - METHOD AND DEVICE FOR MEASURING RADIOACTIVITY. - Google Patents

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method and an apparatus for non-destructively measuring the radioactivity of the contents in a radioactive waste storage container (hereinafter referred to merely as "storage container") such as a drum, and particularly relates to a radioactivity measuring method and an apparatus suitable for easily measuring the radioactivity of γ-ray emitting nuclear material having an unknown density and radioactivity distribution stored in a storage container.

Methods for measuring the radioactivity of the contents of a storage container having an unknown density have been proposed, as disclosed in Japanese Patent Laid-Open Nos. 56-115974 and 61-107183. In the known radioactivity measuring method disclosed in Japanese Patent Laid-Open No. 56-115974, the weight of the contents in a storage container whose radioactivity is to be determined is measured, then an average density is calculated by dividing the thus measured value of this weight by the volume of the container, and the radioactivity in the storage container is measured using the calculated average density. However, there was a problem that this known method could not be applied in the case where the density of the contents changes significantly in an axial direction of the storage vessel.

On the other hand, in the other known radioactivity measuring method as disclosed in Japanese Patent Laid-Open No. 61-107183, the density of the contents in the storage container is obtained using a radiation detector and an external γ-ray source facing each other with a storage container therebetween, and the radioactivity of the contents in the storage container is measured using the density thus obtained. In this case, the measurement is carried out in each of the cross sections (of the container) having different axial heights determined by a collimator arranged in front of the radiation detector, and the total radioactivity in the storage container is determined by the result of these measurements. Accordingly, in this known method, in order to distinguish the γ-ray emitted by the contents in the container from the γ-ray emitted by the external γ-ray emitting source, two radiation measurements with and without the external γ-ray emitting source must be carried out for each of the predetermined cross sections of the container, thereby causing a problem that it requires a relatively long time to measure the total radioactivity in the storage container.

EP-A-0114968 discloses a radioactivity measuring device comprising a detector of the radiation emitted by an object to be measured; means for determining energy spectra; means for calculating an intensity of scattered and unscattered rays; means for calculating a spectrum index as a density index of the object; and means for determining the radioactivity of the object on the basis of the density related value and the strength of the unscattered rays.

EP-A-0 193937 discloses a radioactivity measurement in which a distribution of radiation coefficients is obtained for each net of an object to be tested using the so-called CT principle and the radioactivity is then obtained for each net. In this measurement, a CT device is required and accordingly a large-sized measuring device is required for measuring radioactivity. Further, sometimes a complete measurement cannot be carried out with only one measuring process cycle.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and an apparatus for measuring radioactivity which can measure the radioactivity of the contents in a storage container in a short time with a measurement for each of the predetermined cross sections of the container, even when the density of the contents in the container is unknown.

Another object of the present invention is to provide a method and an apparatus for measuring radioactivity which can measure the radioactivity in a storage container and determine the density of the contents in the container with a simple structure.

These objects are achieved according to the present invention by a radioactivity measuring method as claimed in claims 1 and 2 and by a radioactivity measuring device as claimed in claims 6 and 7.

Advantageous further features are claimed in subclaims 3 to 5 and 8 to 9.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a radioactivity measuring device according to a preferred embodiment of the present invention;

Fig. 2 is a view of a portion of the device shown in Fig. 1;

Fig. 3 is a diagram showing how to determine the intensities of the scattered rays and intensities of the unscattered rays on the wave height distribution of the energy spectra measured by a radiation detector in the radioactivity measuring device;

Fig. 4 is a diagram for illustrating a relationship between a spectrum index and a density-related value in the case where the density-related value is the average density of the object to be measured;

Fig. 5 is a flow chart illustrating a calculation process for determining the average density from the calibration curve of Fig. 4 in a density-related value calculation system;

Fig. 6 is a diagram showing how to determine the intensity of unscattered rays and intensity of scattered rays in a radioactivity measuring device according to another embodiment of the present invention;

Fig. 7 is a diagram for illustrating a relationship between a spectrum index and a density-related value in the case where the density-related value is a ratio between the radioactivity and the strength of the unscattered beam; and

Fig. 8 is a diagram showing how to determine the intensities of the unscattered rays and the intensities of the scattered rays in a radioactivity measuring apparatus according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be explained in connection with its preferred embodiments with reference to the accompanying drawings.

Fig. 1 and 2 show a radioactivity measuring device according to a first embodiment of the present In this embodiment, a drum 1 is used as a storage container which is an object to be measured, and for the sake of brevity of explanation, it is assumed that the drum contains only a nuclide which emits γ-ray having a monochromatic energy E₀.

In Figs. 1 and 2, the drum 1 as an object to be measured can be rotated in a direction R by means of a suitable rotating means (not shown) and can also be raised and lowered in the axial direction L of the drum by means of a suitable lifting means (not shown). The γ-ray emitted from the interior of the drum 1 is measured by a radiation detector 2. The radiation detector 2 can be, for example, a NaI (Tl) scintillator. A vertical collimator 4 is arranged in front of the radiation detector 2 in such a way that the detector 2 can detect or measure the γ-ray emitted by the radioactivity in a cross section 3 in the longitudinal or axial direction L of the drum. Furthermore, a horizontal collimator 5 is provided in front of the radiation detector 2 for correcting the detection efficiency for the position of the γ-ray emitting core material in the cross section 3.

An output from the radiation detector 2 is transmitted to a multi-channel pulse height analyzer 6 which determines energy spectra based on unscattered rays 7 emitted in the drum 1 and passing through the drum 1 and scattered rays 8 which are scattered once in the drum and thereafter pass through the drum. An output of the multi-channel pulse height analyzer 6 is transmitted to a light peak processor 9, which calculates the corresponding count rates based on the unscattered rays 7 and the scattered rays 8. These count rates represent the strengths of the unscattered rays 7 and the scattered rays 8. An output from the peak light processor 9 is transmitted to a spectrum index processor 10 where a ratio between the strength of the scattered rays 8 and the strength of the unscattered rays 7 is calculated as an example of a spectrum index. An output from the spectrum index processor 10 is transmitted to a density-related value processor 11 where an average density of the contents in the cross section 3 of the drum is calculated from the spectrum index. Outputs from the peak light processor 9 and the density-related value processor 11 are transmitted to a radioactivity processor 12 which calculates the radioactivity in the cross section 3 on the basis of the counting rate of the unscattered rays 7 and the average density and also calculates a total amount of radioactivity in the drum by adding the values of radioactivity in all of the predetermined cross sections of the drum. Data obtained from the radioactivity processor 12 are output from an output device 13 such as a printer or a CRT.

The light peak value processor 9, the spectrum index processor 10, the density-related value processor 11 and the radioactivity processor 12 may be constructed as a microcomputer.

The operation of the radioactivity measuring device constructed in this way is then explained.

In order to determine the amount of radioactivity of the γ-ray emitting core material in the drum on the basis of the intensity of the γ-rays measured around the drum, the measured intensity of the unscattered γ-rays must be corrected on the basis of the detection efficiency (detection efficiency) of the measuring system depending on the distribution of radioactivity and the distribution of the densities of the contents in the drum. To do this, the measuring system comprising the drum 1, the collimators 4, 5 and the radiation detector 2 is checked at least once before a number of measurements of the radioactivity in the drum are carried out.

The distribution of radioactivity and the distribution of the densities of the drum contents in the longitudinal direction of the drum are corrected by adjusting the aperture of the vertical collimator 4 in such a way that the radiation detector 2 observes only one cross section of the drum in its longitudinal direction, and by measuring all given cross sections of the drum while gradually raising the drum in the longitudinal direction of the drum.

On the other hand, the correction in the cross section of the drum is effected as follows: Regarding the distribution of radioactivity, the aperture l₀ of the horizontal collimator 5 can be set so that the detection efficiency with respect to the unscattered rays measured by the radiation detector 2 is constant regardless of the positions where the γ-rays are emitted. Accordingly, the measurement is carried out using this aperture l₀. The optimum aperture l₀ is by preparing a drum containing a reference substance having a known density. The reference substance may be, for example, water, cement, air or the like. With respect to the density distribution of the contents in the drum, the correction is made so that the radioactivity can be accurately measured by selecting an optimum aperture width l₀ for the horizontal collimator 5 and by previously determining a relationship between the density of the contents or a value related to the density of the contents and a spectrum index as a calibration curve.

As mentioned above, after the measuring system is set up, the radioactivity in each of the cross sections of the drum is measured in the following manner.

First, in the multi-channel pulse height analyzer 6, the energy spectrum of the γ-ray measured by the radiation detector 2 when the drum 1 is rotated by one revolution by means of the rotating means is obtained. The distribution of the pulse heights or these energy spectra varies according to the density of the contents in the drum 1. When the drum 1 contains only one nuclide capable of radiating γ-rays with a monochromatic energy E₀, the distribution of the energy spectra becomes as shown in Fig. 3. Generally, as the density of the contents increases, the γ-rays are more easily scattered, and the relative value of the intensity C of the scattered rays becomes stronger relative to the intensity of the unscattered rays, while the intensity P of the unscattered rays becomes smaller. If the energy spectra are divided by the light peak value of the energy E₀ of the unscattered rays entering the radiation detector 2, the distribution of the energy spectra becomes as shown by a dashed line in the case of a high density H of the content and becomes as shown by a solid line in the case of a low density L. In Fig. 3, a spectrum based on the unscattered rays in a light peak region is around the energy E₀, and if an energy range ΔE1 including the energy E₀ in this light peak region is given, the sum P of the counting throughputs in this energy range ΔE1 corresponds to the strength of the unscattered rays 7. The energy range ΔE1 can be selected, for example, for the energy resolution of the radiation detector 2. Accordingly, in the light peak processor 9, by setting the energy range ΔE1 including the energy E₀ in the light peak region, the sum P of the counting throughputs in this region is obtained, and therefore the strength of the unscattered rays 7 is calculated.

Further, in Fig. 3, regions other than the light peak region are due to the components of the unscattered rays 7 scattered in the radiation detector 2 and the scattered rays scattered in the cross section 3 of the drum. The former components are inherent in the radiation detector 2, and therefore the ratio of the strength of the components to the strength of the unscattered rays does not change even though the density of the contents in the drum changes. Accordingly, if an energy range ΔE2 is given in a certain region, for example, in a region below and immediately adjacent to the light peak region, lower than the light peak region, the sum C of the counting throughputs in this energy range ΔE2 of the intensity of the scattered rays 8. Likewise, the energy range ΔE2 can be chosen, for example, for the energy resolution of the radiation detector 2. In this way, the photo peak processor 9 can calculate the intensity C of the scattered rays 8.

The spectrum index processor 10 calculates, as a spectrum index, a ratio C/P between the intensity C of the scattered rays and the intensity P of the unscattered rays on the basis of the data from the light peak processor 9.

As can be seen in Fig. 3, the spectrum index C/P depends on the content in the cross section 3 of the drum and varies as shown in Fig. 4. Fig. 4 shows a relationship between the density of the content and the spectrum index obtained when the γ-ray source with a given radioactivity is placed in the drum and the density of the content in the drum is varied. The density-related value processor 11 stores therein the following calibration equation (1) representing the relationship between the density of the content and the spectrum index C/P as shown in Fig. 4:

= f(C/P) (1)

Fig. 5 shows a process flow chart in the case where the density-related value processor 11 is constituted by a microcomputer. Along this process flow chart, the density of the content can be obtained. In particular, the spectrum index C/P is calculated from the sums P and C of the counting throughputs in the energy ranges ΔE1 and ΔE2, then the density of the contents is calculated from the spectrum index C/P thus obtained and the above-mentioned calibration equation (1), and the value of the density is fed to the radioactivity processor 12.

In the radioactivity processor 12, the amount of radioactivity in each cross section 3 of the drum is calculated from the outputs of the light peak value processor 9 and the density-related value processor 11 in the following manner.

If the radioactivity in a drum cross-section is 3 A, the radioactivity A and the intensity P of the unscattered rays generally satisfy the following relationship (2) in approximation:

P = ηA exp (-µm t) (2),

where µm is a mass attenuation coefficient of the γ-rays depending on the energy, which, however, does not depend on the type of substance in the case of γ-rays with an energy level above about 300 kev; t is an average distance that the γ-ray passes through in the drum cross-section 3 and which can be chosen, for example, like the radius r of the drum 1. Further, η is a detection sensitivity which is one of the components of the detection efficiency. The detection sensitivity η can be shown by the product of the geometric efficiency and the intrinsic efficiency of the radiation detector which depends mainly on the energy of the unscattered rays and is maintained at a substantially constant value even when the density of the contents varies. When the apertures of the vertical and horizontal collimators are determined, the geometric efficiency becomes constant. Thus, using the measuring system with adjusted collimator apertures, a nuclear material having the known radioactivity A₀ is added to a reference material having a density ₀, and the intensity P₀ of the unscattered rays passed through the material is measured. When the intensity P₀ is determined, the detection sensitivity η can be calculated from the following equation (3) which is obtained by modifying the above-mentioned equation (2):

η= P₀/A₀ exp (µm P₀r) (3)

Accordingly, if the detection sensitivity η for the energy E of the unscattered rays is directly calculated according to different nuclear materials, the following calibration equation (4) can be obtained in advance:

η= g(E) (4)

The calibration equation for the detection sensitivity η, the mass attenuation coefficient µm and the average transmission path r of the beam are stored in advance in the radioactivity processor 12. The radioactivity processor 12 first calculates the detection sensitivity η from the energy E of the unscattered rays supplied from the light peak processor 9 and the calibration equation (4). Then, the radioactivity A is calculated from the detection sensitivity η thus obtained, the intensity P of the unscattered rays supplied by the light peak value processor 9 and the density of the content supplied by the density-related value processor 11, calculated using the following equation:

A = P exp (µm r)/η (5)

In this way, the radioactivity processor 12 calculates the amount of radioactivity in the drum cross-section 3. Thus, the radioactivity processor 12 can calculate the total amount of radioactivity in the drum by calculating the radioactivity of each cross-section of the drum 1 and then adding all the calculated values of the radioactivity of all the drum cross-sections.

As explained above, according to the present invention, when measurement conditions such as the apertures of the vertical collimator and the horizontal collimator, the detection sensitivity, the average transmission path of the rays, and the like are once determined or set, the amounts of radioactivity in the various cross sections of the drum can be determined simply by measuring the γ-rays emitted from the inside of the drum once for each cross section of the drum, thereby shortening or reducing the measurement time. Further, since the density of the contents in the drum can be obtained without using an external γ-ray source, the apparatus can be simplified.

While the present invention has been explained in the above-mentioned embodiment that only one core material contained in a drum, the present invention can be applied to the case where a plurality of nuclides are contained in a drum.

Hereinafter, the present invention will be explained in connection with a second embodiment in which the drum contains a plurality of nuclides with reference to Fig. 6. Fig. 6 shows the distribution of γ-ray spectra measured by the radiation detector 2 in the case where the drum contains a plurality of nuclides, where E₀₁, E₀₂ and E₀₃ represent the energies of the unscattered rays 7 generated by the individual nuclides. In the light peak processor 9 according to the second embodiment of the present invention, the strength Poi (i = 1, 2, 3, ...) of the unscattered rays of the respective nuclides and the strength C₀₁ of the scattered rays of the associated nuclear materials with respect to the highest energy E₀₁ are calculated as follows. First, in the same manner as that in the case where the drum contains only a core material, as shown in Fig. 3, the intensity P₀₁ of the unscattered rays and the intensity C₀₁ of the scattered rays are calculated for a nuclide having the highest energy E₀₁ of the unscattered rays. By drawing a smooth curve or a straight line (shown by a dashed line in Fig. 6) on the thus obtained spectra tangential to the light peak regions, the intensity P₀₂, P₀₃ of the unscattered rays having an energy below E₀₁ are obtained as the sum of the counts or the area over the curve or straight line. While the spectra of the scattered rays emitted from the inside of the container to be measured are little prominent, the spectra of the unscattered rays have sharp shapes, which show, for example, a Gaussian distribution. It can be considered that the values of P₀₂, P₀₃ thus obtained depend essentially on the strength of the unscattered rays.

Thereafter, in the spectrum index processor 10, the spectrum index C/P is calculated from the intensity C₀₁ of the scattered rays and the intensity P₀₁ of the unscattered rays calculated in the light peak processor 9, taking into account the highest energy E₀₁ of the unscattered rays alone, as shown in Fig. 6.

The density-related value processor 11 calculates the density of the content from the above spectrum index according to the process flow chart of Fig. 5.

The radioactivity processor 12 first calculates the detection efficiency ηoi for each energy Eoi of the unscattered rays of the various nuclides from the value of the strength Poi of the unscattered rays and from the equation (4) stored in the processor 9. Then, the processor 9 calculates the amounts of radioactivity Aoi (i = 1, 2, 3, ...) of the various nuclear materials from the thus obtained detection efficiency ηoi of the strength Poi of the unscattered rays and the density of the contents obtained by the processor 11 of the density-related value using the equation (5). In this case, although the mass attenuation coefficient µm of the rays is constant as in the case where the drum contains only one nuclear material, the mass attenuation coefficient of the rays can be changed to To improve the accuracy of the measurement, the energy E of the unscattered rays can be determined by setting up a calibration equation for the energy E of the unscattered rays.

In this way, according to the present invention, even if the drum contains a plurality of nuclides, the radioactivity for each nuclear material can be obtained.

In the above-mentioned embodiment, the density-related value processor 11 calculates the density of the contents. However, in some cases, there is no need to calculate the density of the contents. Therefore, an explanation will now be given of a third embodiment of the present invention in which the radioactivity in the drum is measured using a value related to the density of the contents in the drum without calculating the density of the contents.

In equation (2), when the energy of the γ-rays is 300 kev or more, since the mass attenuation coefficient µm almost does not depend on the density of the contents and the average path t of the rays penetrating the drum cross-section is equal to the radius r of the drum (i.e. is constant), the exp (-µm pr) directly corresponds to the density of the contents. Thus, the following equation can be considered for the value related to the density:

F = exp (µm r)/η (6)

Using the above-mentioned equation (2), the value F relating to the density of the content is rewritten as follows:

F = A/P (7)

Then, first, a nuclear material having a known radioactivity A₀ is added to the content having an unknown density, and the intensity P₀ of the unscattered rays and the intensity C₀ of the scattered rays generated in such content are measured. Then, the spectrum index C₀/P₀ and the density-related value F₀ are calculated from these intensities P₀, C₀ and the radioactivity A₀, and then the relationship between the spectrum index C₀/P₀ and the density-related value F₀ is determined. After determining such a relationship for each of the different contents with various different unknown densities, it is possible to obtain the following calibration equation (8) which represents a relationship between the density-related relation value F and the spectrum index C/P, as shown in Fig. 7:

F = h(C/P) (8)

Further, by modifying the above-mentioned equation (7), the following equation can be obtained:

A = PF (9)

For a drum containing contents of unknown density, it is possible to determine the amount of radioactivity such contents on the basis of the strength P of the unscattered rays and the density-related value F obtained from the spectrum index C/P. This embodiment has advantages that it is not necessary to determine in advance the detection sensitivity η for each of the different core materials and that there is no need to measure the density of the contents when the equation (8) is obtained.

In the above-mentioned first and second embodiments, the intensity P of the unscattered rays is in the light peak region, and the intensity C of the scattered rays is in the region below the light peak region; however, the present invention is not limited to this case.

Now, another embodiment (fourth embodiment) of the present invention in which the intensity C of the scattered rays and the intensity P of the unscattered rays are adjusted in a different manner will be explained in connection with Fig. 8. In this embodiment, it is assumed that the drum (object to be measured) contains the core material which radiates γ-rays having a monochromatic energy E₀. Fig. 8 shows the distribution of energy spectra of such γ-rays obtained from the output of the radiation detector 2 by the multi-channel pulse height analyzer 6. In Fig. 8, a line m is a straight line connecting a starting point and an ending point of the light peak range in the distribution curve of wave heights. In this case, if an energy range ΔE including the energy E is given, the intensity P of the unscattered rays is represented by an area above the line m in the energy range ΔE within the light peak range, and the intensity C of the scattered rays is represented by an area other than the aforementioned area in this energy range. The energy range ΔE may be optional.

In the above-mentioned explanation, the single nuclide is described. However, this fourth embodiment can also be applied to the case where the drum contains a plurality of different nuclides by adopting the same consideration as described in the second embodiment.

In the fourth embodiment, the intensity P of the unscattered rays and the spectrum index C/P are determined on the basis of such areas divided by the line m by means of the light peak value processor 9 and the density-related value processor 11.

Further, in the first to fourth embodiments, the ratio C/P between the intensity C of the scattered rays and the intensity P of the unscattered rays is used as the spectrum index; however, the spectrum index is not limited to such a ratio. As generally explained mainly with reference to Fig. 3, as the density of the content increases, the γ rays are more easily scattered, and the intensity C of the scattered rays becomes higher and the intensity P of the unscattered rays becomes lower. By normalizing the intensity C of the scattered rays with the energy E₀ or intensity P₀ of the unscattered rays which the radiation detector 2, the intensity C of the scattered rays becomes relatively weak in the case of a high density of the contents, but relatively high in the case of a low density of the contents. This means that the spectrum index for determining the value related to the density requires the ratio C/P between the intensity C of the scattered rays and the intensity P of the unscattered rays or requires containing the ratio C/P as a principal constituent term. Accordingly, the spectrum index may be in any form if it is a function of the ratio C/P or P/C.

As mentioned above, according to the present invention, a method and an apparatus for measuring radioactivity can be provided which can measure the radioactivity in the storage container in a short time by a measuring step for each predetermined cross section of the container even when the density of the content in the container is unknown.

Furthermore, the present invention can provide a method and an apparatus for measuring radioactivity which can measure the radioactivity in the storage container and determine the densities of the contents in the container with a simple structure.

In addition, the present invention can provide a method and apparatus for measuring radioactivity that can measure the radioactivity in the storage container without determining the density of the contents in the container.

Claims (9)

1. Radioactivity measurement method comprising the steps: Determining the energy spectra of radiation rays that are detected by a radiation detector (2) through a collimator (5) mounted in front of the radiation detector in such a way as to obtain a distribution of the energy spectra; Calculating a spectrum index from strengths of scattered (8) and unscattered rays (7) determined by the distribution of the energy spectra; Calculating a density of an object to be measured (1) from a relationship between this density and the spectrum index; and Calculating the radioactivity of the object (1) from the density and the strength of the unscattered rays (7). 2. Radioactivity measurement method comprising the steps: Determining the energy spectra of radiation rays that are detected by a radiation detector (2) through a collimator (5) mounted in front of the radiation detector in such a way as to obtain a distribution of the energy spectra; Calculating a spectrum index from strengths of scattered (8) and unscattered rays (7) determined by the distribution the energy spectra are determined; calculating a value relating to a density of an object to be measured (1), which value is a function of a ratio between radioactivity and strength of the unscattered rays, from a relationship between the value relating to the density and the spectrum index; and Calculating the radioactivity of the object (1) from the value related to the density and the strength of the unscattered rays. 3. Method according to claim 1 or 2, wherein the strength of the scattered rays (8) results from a sum of count rates in a predetermined energy range under a photo peak range and the strength of the unscattered rays (7) results from a sum of count rates in a predetermined range in the photo peak range. 4. A method according to claim 1 or 2, wherein a photopeak region is divided by a straight line connecting a starting point to an end point of the photopeak region on a distribution curve of wave heights, and the intensity of the scattered rays (8) results from a sum of count rates in a predetermined energy range in the photopeak region excluding a part above the straight line, while the intensity of the unscattered rays (7) results from a sum of count rates in a predetermined energy range in the photopeak region in the part above the straight line. 5. Method according to claim 1 or 2, wherein the object to be measured is a content in a drum (1) which is arranged for rotation and up and down movement by a drive mechanism. 6. Radioactivity measuring device comprising: a radiation detector (2) for detecting the radiation rays emitted by an object to be measured; a collimator (4; 5) mounted in front of the radiation detector (2); Means (6) for determining an energy spectrum of the radiation rays; Means (9) for calculating an intensity of scattered rays (8) and an intensity of unscattered rays (7) on the basis of the distribution of the energy spectrum; Means (10) for calculating a spectrum index which is determined by the intensity of the scattered rays (8) and the intensity of the unscattered rays (7); Means (11) for calculating the density of the object (1) to be measured from a relationship between the density and the spectrum index; and Means (12) for calculating the radioactivity of the object to be measured (1) from the density and the strength of the unscattered rays (7). 7. Radioactivity measuring device comprising: a radiation detector (2) for detecting the radiation rays emitted by an object to be measured; a collimator (4; 5) which is arranged in front of the radiation detector (2) is appropriate; Means (6) for determining an energy spectrum of the radiation rays; Means (9) for calculating an intensity of scattered rays (8) and an intensity of unscattered rays (7) on the basis of the distribution of the energy spectra; Means (10) for calculating a spectrum index which is determined by the intensity of the scattered rays (8) and the intensity of the unscattered rays (7); Means (11) for calculating a value relating to a density of the object (1) to be measured, which value is a function of a ratio between radioactivity and strength of the unscattered rays, from a relationship between the value relating to the density and the spectrum index; and Means (12) for calculating the radioactivity of the object to be measured (1) from the value related to the density and the strength of the unscattered rays (7). 8. Device according to claim 6 or 7, wherein the object to be measured is a drum (1) including a drive mechanism for rotating, raising and lowering the drum (1). 9. Device according to claim 6 or 7 for measuring a radioactivity of a drum (1), which has a drum drive mechanism with a mechanism for rotating, raising and lowering the drum.